Various types of information display devices have been generally used such as CRT (cathode ray tube), fluorescent character display tube, EL (electroluminescence) displays. Recently, the liquid crystal displays which offer a thin structure and a low power consumption enjoy growing popularity in the field of word processors and notebook personal computers. A variety of formats (display modes) of the liquid crystal displays have been proposed to find ways to achieve more effective use of the liquid crystal displays as display devices.
For example, in a DAP (deformation of vertical aligned phases) mode, liquid crystal molecules are vertically aligned to a substrate surface using a liquid crystal having a negative .DELTA..epsilon. (dielectric anisotropy), and liquid crystal cells are placed under cross nicol. In this arrangement, with the application of a threshold voltage or above threshold voltage to the liquid crystal cells, a molecular axis is tilted with respect to an electric field, and an optical axis of the liquid crystal molecules is displaced from the polarization axis. As a result, a double-refraction effect is generated, and thus light is transmitted. On the other hand, when a voltage is not applied, apparently, a single refraction index exists. Thus, light transmitted through the first polarizer is completely shut off by the second polarizer and thus it is not transmitted. As described, when a voltage is not applied, a transmittance is very low, thereby achieving a high contrast display in the DAP mode.
Whereas, only by simply aligning the liquid crystal molecules vertically to the substrate surface, the liquid crystal molecules are randomly tilted with the application of a voltage. As a result, many areas having different tilt angles of the molecules are formed. The areas present not only the problem of different transmittances but also present the problem of unsuccessive orientation of the molecules which causes dispersion of light on a boundary between areas. In order to prevent the described problems, it is required to align the liquid crystal molecules beforehand so as to be tilted from a direction of a substrate normal. Furthermore, if this tilt angle (hereinafter referred to as a pretilt angle) becomes too large, light may be transmitted by the double refraction effect as generated with an application of the voltage, which causes the contrast to be lowered. Therefore, it is preferable to set the pretilt angle as small as possible.
In order to achieve the described orientation, conventionally for example, a method in which vertical orientation process is carried out after subjecting a polyimide film (parallel orientation film) to a rubbing process is known as disclosed in Japanese Laid Open Patent Publication No. 167123/1981 (Tokukaisho 56-167123). According to this method, after rubbing the surface of a polyimide high polymer, the substrate is dipped in a solution of silane surface active agent dissolved in an organic solvent, and the substrate is calcined, thereby obtaining a film of the silane surface active agent. This method offers a pretilt angle of 2.degree.-4.degree..
Whereas the method disclosed in the U.S. Pat. No. 919155 offers a pretilt angle of 0.5.degree.-6.degree. which is an effective orientation method in the DAP mode. According to this method, after evaporating SiO in a direction forming an angle of 30.degree. with respect to the substrate plane, the substrate is rotated at 90.degree. so as to carry out the second evaporation in a direction forming an angle of 5.degree. with respect to the substrate surface, and the substrate is placed under long-chain alcohol which is heated to 140.degree. C. for two hours.
Other than the DAP mode, liquid crystal displays of the TN (twisted nematic) mode, STN (super twisted nematic) mode, etc., have been practically used in a variety of fields. However, described formats present a deficiency that the display can be seen differently depending on a viewing angle (viewing angle characteristic). The various techniques for achieving improvements in the viewing angle characteristic have been proposed such as a method for varying a threshold voltage among picture elements and a method for varying the direction of the orientation of the liquid crystal molecules and the size of the pretilt angle per every picture element (see Kalluri et al. 1993 Society for Information Display International Symposium (SID'91), Y. Koike et al. 1992 Society for Information Display International Symposium (SID'92), K.Takatori et al. Japan display'92).
For the described reasons, the pretilt angle is required to be made as small as possible in the DAP mode. Therefore, when the DAP mode is applied to liquid crystal projection which especially requires high contrast, it is necessary to set the pretilt angle to 1.degree. or below. Therefore, it is not feasible with the method disclosed in the Japanese Laid-Open Patent Publication No. 167123/1981 (Tokukaisho 56-167123).
The method disclosed in the U.S. Pat. No. 919155 offers a pretilt angle of 0.5.degree.. However, the evaporation process adopted in this method is a so-called oblique evaporation, which requires a vacuum condition. Moreover, the method presents the problem that it is difficult to uniformly align a depositing angle of SiO molecules on an entire surface of the substrate, which results in the deterioration of display evenness due to the variation in the pretilt angle. As described, there has not been found the way to apply the method on an industrial basis.
The conventional techniques for achieving improvements in the viewing angle characteristics are complex by requiring the following processes:
(1) rubbing an orientation layer, masking part of the orientation layer with a resist after carrying out the orientation treatment, and removing the resist after carrying out the second rubbing process at an angle different from the first rubbing process; and PA1 (2) forming orientation layers of two kinds having different pretilt angles by repeating a photolithographic printing process twice.
As described, the described manufacturing processes are complex because the photolithographic printing process is required once or twice. Moreover, the process is carried out on the orientation layer which the liquid crystal is in direct contact with. Therefore, the problem is presented in that the pretilt angle varies due to the photo resist and separation liquid used in the photolithographic printing process. In the case of applying the described methods to the TFT (thin film transistor) liquid display, an adverse effect occurs such as a drop in the voltage holding ratio, etc., due to the residues of the photo-resist and the separation liquid. Furthermore, the control of the pretilt angle is limited by the materials for the liquid crystal and the orientation layer.
As described, any of the described techniques fail to offer an accurate and easy control of the pretilt angle. Therefore, it is difficult to achieve a high contrast display and improvements in the viewing angle characteristic.